Wavelength division multiplexed (WDM) optical communication systems are known which combine multiple modulated signals at different wavelengths onto a single optical fiber. Typically, the MZ modulators in such systems are discrete components which are housed separately from other components, such as lasers and optical combiners. Recently, however, optical components for WDM applications have been integrated onto a common chip or semiconductor substrate. In particular, so-called “photonic integrated circuits” (PICs) have been developed which have multiple optical paths, each of which including a series of optical components. For example, an optical path may include a laser, modulator, optical amplifier, and variable optical attenuator. The optical signals supplied from each optical path may be supplied to an optical combiner, which combines or multiplexes each optical signal onto a common output.
Mach-Zehnder (MZ) modulators are commonly used in optical communication systems in order to modulate an optical signal in accordance with information to be transmitted to a receiver. A Mach-Zehnder modulator typically includes an interferometer having an input waveguide and arms that branch from the input waveguide. An output waveguide is also provided at the junction of the arms. Typically, an optical signal is directed into and propagates in the input waveguide, and is split between the arms so that approximately one-half of the input optical signal propagates in each of the interferometer arms. A modulating signal is applied to an electrode adjacent one or both of the arms in order to change the effective refractive indices in the interferometer arms and to introduce a relative phase shift between the two optical signals. The phase-shifted optical signals combine at the output waveguide and may produce intensity modulated optical signals. Depending on the relative phase shift between the two optical signals, they may interfere either constructively or destructively. The output of the modulator may thus be an intensity modulated optical signal. A relative phase-shift between the optical signals in the arms of approximately π is required to switch the output of the modulator between adjacent on and off states, and the modulation voltage required to achieve such a phase shift is referred to as Vπ. The output of the modulator may also be phase modulated, as well.
If multiple modulators are provided on a PIC, Vπ may vary for each optical signal path, such that modulators in some optical signals paths may have associated Vπ values that are greater than Vπ values associated with modulators in other optical paths. In that case, circuitry provided on the PIC for driving the modulators may not be able to supply the entire range of Vπ values.
Thus, there is a need for a PIC having a plurality of driver circuits that supply substantially the same Vπ to corresponding MZ modulators or supply Vπ values that are less than a predetermined voltage. There is also a need to optimize the performance of each optical path.